The present invention pertains to an apparatus for applying an encircling binding strap to a package being conveyed through a system and, in particular, to a system for strapping a compressible stack of sheets of corrugated paperboard by applying a strap around the stack in the direction of its movement through the system.
Various kinds of apparatus for banding or strapping packages being conveyed through a system are known in the art. In particular, packages comprising vertical stacks of sheet material, such as corrugated paperboard, may be secured by banding with a metal or plastic strap or tieing with cord or twine. Further, various systems are known for banding or tying such stacks automatically upon receipt from an upstream stacking apparatus and prior to palletizing or unitizing.
Specifically, one prior art system is utilized for strapping stacks of corrugated paperboard blanks used to construct cartons or boxes. The corrugated blanks or knocked-down cartons are formed in a so-called flexo-folder-gluer and, after formation, a specified number are automatically stacked and ejected into a strapping system. The strapping system typically includes a powered in-feed conveyor and a mechanism for squaring the stack and delivering it to a strapping station. After strapping, the stack is conveyed from the system for further processing, such as automatic unitizing with a plurality of similarly strapped stacks. A number of common problems have made the construction and operation of prior art strapping systems less than desirable. First of all, because of the wide variation in the size and shape of the corrugated sheets pre-formed in the flexo-folder-gluer, the centerline of a stack of such sheets or knocked-down cartons coming into the strapping system may be offset substantially from the centerline of the system. Thus, prior art strapping machines have typically been constructed to be movable laterally to place the centerline of the machine approximately coincident with the centerline of the stack of cartons being run at a particular time. Obviously, this requires a repositioning of the strapping machine each time blocks of different size corrugated sheets are run. In addition, lateral movement of the strapping machine will often displace it from the centerline of the downstream conveying equipment receiving the stapped bundles, resulting in further alignment and handling problems.
Because of the need for rapid handling and processing, the unbound stacks of sheets entering the strapping system tend to be out of square and must be squared before strapping so the final strapped bundle is also square and to prevent edge damage to displaced sheets. Typical prior art systems thus include side tamps on the in-feed conveyor to square the lateral sides of the stack and establish the stack generally on the longitudinal centerline through the system. To square the forward and rear faces of the stack, some prior art systems simply rely on a vertically disposed pusher which is automatically positioned behind the stack on the in-feed conveyor to simultaneously square the rear edges of the sheets and push the stack into the downstream strapping position. However, merely pushing the stack from the rear does not assure that the front and rear faces will be squared. In addition, movement of the stack out of the side tamps and into the strapping position often results in momentary loss of positive stack retention, again resulting in loss of stack squareness. After the stack of corrugated sheets is received in the strapping station, it is typically vertically compressed before the encircling strap is applied and, after the strap ends are secured, the compression is released and the expanding stack provides the necessary tension in the strap to secure the stack. Obviously, if the stack is not square at the time it is compressed, the strapped stack will also be out of square. Sometimes it is necessary or desirable to process stacks of knocked-down cartons or the like through the system without strapping. In such cases, failure to maintain or loss of stack squareness will also adversely affect downstream processing.
One prior art device which utilizes a pusher to engage the rear face of the stack and push it into the strapping station, attempts to establish and retain front and rear face squareness by pushing the stack into the rear face of the downstream stack which has just been strapped and to simultaneously push the strapped downstream stack from the strapping station. Nevertheless, there is still a momentary loss of positive stack retention in the transfer from the in-feed conveyor to the strapping station and, in addition, if the downstream strapped stack is out of square, the unstrapped stack pushed into it may be knocked out of square as well.
This same prior art system utilizes a strapping mechanism which holds the free end of a continuous supply of strap below the plane of the stack and an upper intermediate portion above the stack such that the strap end portion lies in a vertical plane in the path of a stack coming into the strapping apparatus. The incoming stack is pushed into the strap, the strap is played out from the continuous supply above, and continuing downstream movement of the stack into the strapping station results in partial wrapping of the stack around its front face and portions of the top and bottom. The upper intermediate strap portion is supported by a hook-shaped arm which is adapted to swing downwardly past the rear of the partially wrapped stack and through a longitudinal slot in the strapping apparatus to carry the intermediate strap portion to a point overlapping the free end held below the stack. The overlapping portion of the plastic strap is heat sealed, the strap is severed to form a new free end which is held below the plane of the bottom of the stack, and the strap arm reverses and swings back upwardly to its upper supporting position, playing out a suitable length of strap which is automatically positioned in the path of the next incoming stack. The system also includes a vertically reciprocable compression plate in the strapping station which compresses the stack just prior to completion of strapping and holds it until the heat sealed connection is made. As indicated, however, this system is characterized by an absence of positive stack retention from squaring through strapping and heat sealing, such that the squareness of the strapped stack cannot be positively assured. In addition, the reciprocating movement of the strap carrying arm requires a complex clamping mechanism in the heat sealing area which must provide for the strap both lateral and longitudinal linear movement, as well as rotary movement through 360.degree. to properly orient the new free end of the severed strap. Finally, the strap cutting mechanism requires rather precise alignment, and loss of alignment can result in serious damage to the heat sealing apparatus.